1. Field of the Invention
The present invention relates to a method of defect evaluation or inspection capable of checking ceramic products of complicated shapes for defects.
2. Discussion of the Background
Ceramic products have the advantages in that they are strong and corrosion resistant under high-temperature condition, and are lightweight. With the recent development of ceramics with improved properties, research has been conducted to replace products made of conventional metal materials with ceramic products. Ceramics have a drawback, however, in that they are brittle and liable to fracture. Therefore, in using ceramic products in place of metallic ones, problems lie in their strength under actual temperature and stress conditions. In ceramic products, the occurrence of fracturing depends on the existence of the defects (especially defects near the surface portions), because fractures occur at the cracks or other defects. Accordingly, there is a demand for the development of a method for nondestructively detecting defects in ceramic products.
Nondestructive evaluating methods for metal products includes an ultrasonic pulse method (hereinafter referred to as UT method) and an acoustic emission method (hereinafter referred to as AE method). However, it is difficult to apply these methods directly to ceramic products. The size of cracks to be detected may be given in millimeters for metallic products, but is 0.1 mm or less for ceramic products. According to the prior art method, therefore, it is impossible to detect defects in ceramic products with high accuracy. Some of the cermaic products to be evaluated may have complicated shapes such as those of turbocharger rotors, not to mention plate-like, cylindrical or other simple configurations. Intricate shapes will result in various awkward situations, so that the incidence of ultrasonic waves and the detection of the reflected waves are difficult. Echoes will also reflect in a diffused manner to produce complex echoes, thereby concealing the existence of echoes of minor defects. Thus, it is hard to check ceramic products having intricate shapes for defects by the UT method.
Moreover, it is difficult to apply mechanical stress to ceramic products of complicated shapes. Therefore, it is also hard to make defect evaluation by the AE method in which mechanical stress is applied to produce acoustic emission.